Crosslinked polymers are useful for many purposes, such as those where structural integrity, resistance to flow, toughness, or durability are important. One significant area of use is in protective or decorative coatings, such as paints. In many such uses (e.g., as paint coatings for electrographic copiers or other electrical apparatus, for aircraft, and for high speed manufacturing apparatus such as photographic coating machines) buildup of electrostatic charges on surfaces coated with such paints could cause serious problems. Many crosslinked polymers having other desirable properties are found to exhibit a relatively high degree of electrical resistivity and thus are susceptible to problems such as undesired accumulation of surface electrostatic charge.
Efforts to lower resistivity (i.e., increase conductivity) of crosslinked polymer compositions have involved various approaches.
One approach involves adding a separate conductivity agent to an otherwise electrically insulative composition comprising a crosslinked polymer to lower the overall resistivity of the composition. However, many conductivity agents (e.g., carbon black or metal particles such as steel or aluminum) can adversely affect other properties of the composition, for example, by imparting undesirable coloration or by changing reflective properties. Other conductivity agents (e.g., some organic compounds containing quaternary ammonium moieties) do not have initial adverse effects, but since they are not permanently anchored in the composition, can evaporate or migrate out of the composition, resulting in a continuous change in the resistivity of the composition over time and possible adverse affects on materials that may come into contact with the migrating agent.
Another approach, which avoids evaporation and migration problems, involves bonding quaternary ammonium conductivity agents or precursors thereof directly into monomers before polymerizing them to form the backbones of the polymer or bonding such agents into the polymer or crosslinking agent before crosslinking. For example, U.K. Pat. No. 2,066,257B, published 3 Aug., 1983, describes epoxy resins having nitrogen atoms as integral parts of the resin backbone. Simultaneous reaction of such an epoxy resin with a crosslinking agent and a quaternizing agent results in crosslinking of the resin and conversion of the nitrogen atoms into quaternary ammonium conductive moieties divalently included in the polymer backbone. U.S. Pat. No. 3,011,918 describes preparing crosslinked vinyl polymers from polymerizable monomers already having been specially prepared having quaternary ammonium moieties bonded covalently and pendently thereto. Also, published abstracts of Japanese published patent applications 59/024706 and 59/024727 (both applications published 8 Feb., 1984) describe polymerizing and crosslinking certain monomers, each of which already contains a quaternary ammonium moiety, to produce conductive crosslinked polymers having such moieties divalently included in the polymer backbones.
However, the schemes described in the above-noted U.K. patent, U.S. patent, and Japanese applications also have drawbacks. They require that specialized polymers or monomers containing the quaternary ammonium moieties or precursors thereof be prepared in separate steps before crosslinking the polymers, thus complicating and lengthening the process of preparation. Furthermore, incorporation of conductive moieties directly into polymer backbones such as in the U.K. patent and Japanese applications, may effect profound changes in other properties of the polymers in addition to resistivity. Also, when using specially modified polymer and monomer starting materials, one has little or no leeway to alter the level of permanent resistivity that the resultant polymer will exhibit after crosslinking; in other words, in order to change the resultant level of resistivity, one must often start at the beginning and fashion new specially modified polymers or monomers to be crosslinked.
The present invention overcomes all of the above-noted prior art problems.